This article is composed of three independent commentaries about the state of ICON principles (Goldman et al., 2021) in the AGU section Paleoclimatology and Paleoceanography (P&P), and a discussion on the opportunities and challenges of adopting them. Each commentary focuses on a different topic: (Section 2) Global collaboration, technology transfer and application, reproducibility, and data sharing and infrastructure; (Section 3) Local knowledge, global gain: improving interactions within the scientific community and with locals, indigenous communities, stakeholders, and the public; (Section 4) Field, experimental, remote sensing, and real-time data research and application.P&P projects can better include ICON principles by directly incorporating them into research proposals. A promising way to overcome the challenges of interdisciplinarity and integration is to foster networking, which will advance our research discipline through the application of ICON.

The shift from denser forests to open, grass-dominated vegetation in west-central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n=2065) and new (n=173) oxygen isotope measurements (δ18O) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity. Oxygen isotope ratios in meteoric water (δ18Op) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ18Op (and clay δ18O) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west-central North America were adapted to a wet-winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.

Lacustrine, riverine, and spring carbonates are archives of terrestrial climate change and are extensively used to study paleoenvironments. Clumped isotope thermometry has been applied to freshwater carbonates to reconstruct temperatures, however, limited work has been done to evaluate comparative relationships between clumped isotopes and temperature in different types of modern freshwater carbonates. Therefore, in this study, we assemble an extensive calibration dataset with 135 samples of modern lacustrine, fluvial, and spring carbonates from 96 sites and constrain the relationship between independent observations of water temperature and the clumped isotopic composition of carbonates (denoted by Δ47). We restandardize and synthesize published data and report 159 new measurements of 25 samples. We derive a composite freshwater calibration and also evaluate differences in the Δ47-temperature dependence for different types of materials to examine whether material-specific calibrations may be justified. When material type is considered, there is a convergence of slopes between biological carbonates (freshwater gastropods and bivalves), micrite, biologically-mediated carbonates (microbialites and tufas), travertines, and other recently published syntheses, but statistically significant differences in intercepts between some materials, possibly due to seasonal biases, kinetic isotope effects, and/or varying degrees of biological influence. Δ47-based reconstructions of water δ18O generally yield values within 2‰ of measured water δ18O when using a material-specific calibration. We explore the implications of applying these new calibrations in reconstructing temperature in three case studies.

During the mid-Holocene (MH: ~6,000 years BP) and Last Interglacial LIG (LIG: ~129,000-116,000 years BP) differences in the seasonal and latitudinal distribution of insolation drove northern hemisphere high-latitude warming comparable to that projected in end-21st century low emissions scenarios, making these intervals potential analogs for future climate change in North America. However, terrestrial precipitation during past warm intervals is not well understood and PMIP4 models produce variable regional moisture patterns in North America during both intervals. To investigate the extent to which the latest generation of models reproduces moisture patterns indicated by proxy records, we compare hydroclimate output from 17 PMIP4 models with networks of moisture-sensitive proxies compiled for North America during the LIG (39 sites) and MH (257 sites). Agreement is lower for the MH, with models producing wet anomalies across the western United States (US) where a high concentration of proxies indicate aridity. The models that agree most closely with the LIG proxies differ from the PMIP4 ensemble by showing relative wetness in the eastern US and dryness in the northwest and central US. An assessment of atmospheric dynamics using an ensemble subset of the three models with the highest agreement suggests that LIG precipitation patterns are driven by weaker winter North Pacific pressure gradients and steeper summer North Pacific and Atlantic gradients. Comparison of this LIG subset ensemble with simulations of future low emissions scenarios indicates that the LIG may not be a sufficient analog for projected, end-21st century hydroclimatic change in North America.

The El Niño Southern Oscillation (ENSO) modulates rainfall amount variability and, by extension, river discharge in the Philippines on seasonal to interannual temporal scales. The El Niño phase (ENP) of ENSO considerably decreases rainfall amounts on a seasonal scale in the western Pacific with varying degrees of heterogeneity expressed across the Philippines. Our understanding of the response in the hydroclimate to ENPs on interannual timescales is still relatively immature. As such, to investigate the hydroclimate response, a composite time series of 29 rainfall and 61 river discharge stations spanning 1901-2020 and 1908-2017 C.E., respectively, and covering the four major climate types in the Philippines were assessed. Our results suggest, regardless of climate type, that river discharge and rainfall data decrease following ENPs. The median response suggests that the decreasing trend can last up to seven years. Further, the hydroclimate response follows either a decreasing trend, if at conception of an ENP, or an increasing trend, if at the termination of an ENP. As water-scarcity becomes an area of immediate concern in an increasingly warming climate, our results have implications for interannual water resource management in this drought-prone tropical archipelago.

High latitude glacierized coastal catchments of the Gulf of Alaska (GoA) are undergoing rapid hydrologic changes in response to climate change and glacial recession. These catchments deliver important nutrients in the form of both inorganic and organic matter to the nearshore marine environment, yet are relatively understudied with respect to characterization of the solute generation processes and total yields. Using multiple linear regression informed by Bayesian Information Criterion analysis we empirically demonstrate how watershed characteristics affect solute generation as represented by concentration-discharge relationships. We find that watershed mean slope and relief control solute generation and that solute yields are influenced most by glacier coverage. We contribute a new flux and concentration-discharge based conceptualization for understanding solute cycles across a hydroclimatic gradient of GoA watersheds that can be used to better understand future watershed responses to rapid hydrologic change.